The present invention relates particularly to a so-called differential cylinder, that is, a cylinder which enables the amount of fluid at the output and its pressure to be varied in dependence on the braking stage, as will be described below and as described, for example in U.S. Pat. No. 4,455,831, GB 359142, DE 2436808, U.S. Pat. No. 2,093,543, DE 3631683 and U.S. Pat. No. 1,958,722.
As is self-evident, braking systems operate at least between a rest condition and a working condition which are defined, respectively, by the absence or by the presence of a predetermined braking force applied to the members of the system by actuating means.
Irrespective of the type of braking system, the transition from the rest condition to the working condition comprises a first stage for taking up clearance and a second actual braking stage.
With reference, for example, to hydraulically-operated braking systems, during the stage for taking up clearance, the travel performed by the actuating means is quite large and the pressure is low. In contrast, during the actual braking stage, the travel performed by the actuating means is quite small and the pressure is high.
With reference to braking systems of the type with disc brakes, a disc is mounted on the hub of the vehicle wheel and a caliper, provided with pads, is mounted on a fixed portion of the vehicle, for example, on the suspension, and is arranged astride the disc. By way of example, for a so-called fixed caliper, the caliper has cylinder-piston units disposed on both sides of the disc so that the respective pistons act on the pads, pressing them against the disc to achieve the desired braking action.
When the braking system is in the rest condition, the pads are disposed at a distance from the disc such as to allow free rotation of the disc and not to give rise to so-called residual-torque phenomena. Upon completion of the braking operation, the pistons are therefore retracted inside the cylinders by a distance, generally known by the term “rollback”, such as to permit the desired detachment of the pads from the disc.
It is therefore clear that, during the transition from the rest condition to the working condition of the braking system, the pistons which act on the pads perform, in the first place, a travel which causes the pads to approach the surface of the disc, that is, a travel during which the distance (the rollback) between the pads and the disc is taken up.
In these braking systems, the cylinder-piston units of the caliper are arranged to be connected to a master cylinder which in turn is connected to a control device, for example, constituted by the brake pedal provided inside the passenger compartment.
The approach travel of the pads in order to exert the braking force on the vehicle wheels translates into an approach travel of the master cylinder, that is, into a travel which is lost for the purposes of the braking action. Similarly, the approach travel of the master cylinder translates into a corresponding travel of the brake pedal, during which the pedal does not generate the desired braking force.
Naturally, the approach travel of the pads, together with the consequent loadless travel of the pedal master cylinder, adversely affect the promptness of the response of the brake and this effect is more noticeable the greater the sensitivity of the user and the higher the performance of the vehicle, as in racing cars or, in any case, vehicles for sports uses.
There is therefore a need, on the one hand, to limit the approach travel of the pedal whilst nevertheless supplying a large amount of fluid to the cylinder-piston units housed in the caliper. This need could be solved by providing a master cylinder with a large-diameter piston. On the other hand, however, there is a need to limit the force which must be applied to the brake pedal in order to achieve a predetermined pressure. This need could be solved by providing a master cylinder with a piston of limited diameter.
These conflicting needs are solved simultaneously by providing a master cylinder, known as a differential cylinder, which has a cylindrical cavity having two portions with different diameters, in each of which a corresponding piston slides.
This cylinder therefore has two chambers with different diameters for performing two distinct functions. During the stage of the approach travel of the pads, the larger-diameter chamber is operative and can supply a large quantity of fluid to the brakes whilst keeping the travel of the brake pedal short. During the actual braking stage, the smaller-diameter chamber is operative and enables high pressures of the fluid to be achieved and large braking forces therefore to be exerted whilst the control force on the brake pedal is kept low. The transition from one function to the other takes place when a predetermined pressure value (for example 5 bar) is reached, and is brought about by the opening of a discharge valve by means of which the larger-diameter chamber is put into communication with ambient pressure, for example, from the brake fluid reservoir. An example of a differential cylinder as described briefly above is given, for example, in U.S. Pat. No. 1,958,722. The larger-diameter chamber is put into communication with the reservoir by means of ducts formed in the structure of the cylinder body. The discharge valve is housed inside the ducts, and hence inside the body of the master cylinder.
Further constructions, in which the discharge valve which renders the larger-diameter chamber inactive is housed inside the pistons, for example, inside the rod of the larger-diameter piston, are described and illustrated in GB 359142, DE 2436808, U.S. Pat. No. 2,093,543 and DE 3631683.
The above-mentioned patents describe differential cylinders in which the smaller-diameter piston has some ducts parallel to the axis of sliding of the pistons. Moreover, the seal between the smaller-diameter piston and the respective chamber is ensured by a lipped seal which adheres to the walls of the chamber as a result of the pressure established therein.
During the stage in which the larger-diameter chamber is active, it is known, for example, as described in the patents cited above, for the brake fluid to pass from the larger-diameter chamber to the smaller-diameter chamber through the ducts formed in the smaller-diameter piston and between the walls and the lipped seal, which is not yet adhering to the walls.
When the “rollback” and the clearance have been taken up, the larger-diameter piston is deactivated by the opening of the discharge valve and the lipped seal of the smaller-diameter piston adheres to the walls of its chamber which is isolated from the larger-diameter chamber, permitting an increase in the pressure which causes the pads to be thrust against the disc in order to exert the braking force on the vehicle.
The transitory stage between the deactivation of the larger-diameter chamber and the activation of the smaller-diameter chamber has to be kept to the minimum since it corresponds to an “idle” period of time which could instead be used for the delivery of brake fluid under pressure.
Moreover, the duration of the transitory stage depends on the promptness of the response of the lipped seal which, unfortunately, is not optimal, because it is also designed so as to be suitable for forming the seal against the walls of the respective chamber. This translates into a sensation in the brake pedal of a “step”, which is particularly disagreeable for sports cars or even racing cars.
A further disadvantage of known cylinders arises when, owing to the pressure established in the smaller-diameter chamber, the lipped seal is extruded through the axial holes in the smaller-diameter piston. Attempts have been made to prevent this problem by inserting a ring between the lipped seal and the respective piston, but the solution is not optimal since it is a further factor which contributes to increase “idle” time.
It is known from U.S. Pat. No. 4,455,831 to produce a differential cylinder provided with valve means which are separate from the lipped seal of the smaller-diameter piston and which put the two chambers into communication.
Finally, a two-part piston inside which two separate valves are housed is known from U.S. Pat. No. 3,667,229. A first valve is disposed in a duct inside the piston, which connects the smaller-diameter chamber with the larger-diameter chamber. A second valve is disposed in a duct which connects the larger-diameter chamber with an environment that is in communication with the reservoir. The piston is provided in two portions and each houses a respective valve.
The two valves are separate from one another and each has to be mounted, separately from the other, inside the respective portion of the piston. This leads to problems in the production of the two portions, both of which have to be precision machined in order to house the respective valve correctly, and to problems during assembly, in particular rendering assembly quite complex.
The object of the present invention is to devise and to provide a master cylinder for a vehicle braking system which permits differential operation and, at the same time, overcomes the disadvantages mentioned with reference to the prior art.